Piezoelectric transducers are in common usage in numerous products. They contain a diaphragm typically fabricated by attaching a smaller diameter thin piezoceramic disk onto a larger diameter thin metal disk. Applying a voltage across the piezoceramic disk produces stresses that cause the diaphragm to flex like a drum skin. By energising such a system at an audible sound frequency, sound can be generated. Diaphragms may be used on their own, as in wrist watches, but they are usually mounted onto an acoustic chamber with an opening to improve the acoustic power output of the transducer. Acoustic power output and/or sound directionality can further be enhanced with a suitably shaped air cavity that behaves like a horn.
One common method of mounting a diaphragm is to glue its nodal circle to a matching cylindrical protrusion that forms part of the acoustic chamber. The nodal circle's length does not change as the diaphragm flexes. Nodal mounting is inexpensive and has low mounting loads but exposes only the diaphragm surface within the nodal diameter to the acoustic chamber. Another common method is to mount the diaphragm by gluing or clamping it around a small peripheral annular region. This has higher mounting loads but exposes a larger portion of the diaphragm to the acoustic chamber and equivalent diaphragm flexing results in the centre deflecting further compared with nodal mounting.
One limiting factor of the acoustic power output is the tensile strength of the piezoceramic. Ceramics are typically much stronger in compression than in tension. Increasing the flexing vibration of the diaphragm by applying higher voltage waveforms increases acoustic power output. The fatigue life of the transducer depends mainly on the maximum tensile stress experienced by the piezoceramic. Specifying a required fatigue life for a transducer in turn determines the maximum vibrating deflection, the maximum driving voltage waveform and the maximum acoustic power output for a given frequency.
Previously published proposals in the field of transducers include U.S. Pat. No. 6,353,277 (Han-Jose), U.S. Pat. No. 5,514,927 (Tichy) and U.S. Pat. No. 5,030,872 (Boehnke & Pieper). These specifications are representative of particular configurations of transducers for particular purposes but in recognising this prior art no admission is made that any one item discloses any arrangement which is known or of general knowledge in Australia or any other country.
One important application envisaged for transducer arrangements embodying the present invention is to the field of sound generators and especially those intended to provide high volume output for alarms and applications, such as for use by referees in sporting events. However use of the invention in other applications and fields is envisaged.